1. If you want to have share your workspaces with external users go to your SharePoint Online Central Administration and press the settings hyperlink.
2. Select the maximum sharing level that you want for your workspaces.
3. Press "OK".
4. To change or activate the external sharing for a consisting tempalte you have to navigate to the admin dashboard.
5. Select "Setup".
6. Press "Settings" and then the hyperlink "Permissions"
7. Click on "Workspace Sharing Permission".
8. You can see a list of your templates and the associated sharing level. If you created the template with external sharing option enabled it will be listed here with Sharing Level "Enabled". Press "Edit" for the template that is used at your workspaces which should have external access enabled.
9. The sharing level is set to "External Users". Your workspaces get this level once you create the workspace item or sync it.
External Access for single Workspace
1. If you dont want to share all workspaces of a template but a single one you first have to go to your SharePoint Online Central Administration and press the settings hyperlink.
2. Select the maximum sharing level that you want for your workspace.
3. Press "OK".
4. Go back to your Collaboration Manager 365 SiteCollection and press the "Workspaces" hyperlink. Press on the LIST Tab and create a new column.
5. Name it "ExternalSharing", set the type to "Yes/No (check box)", set the Default value to no and add it to all content types. Press the "OK" button.
6. The column is added to the list. If you add an item or update it and the field is checked the workspace gets the sharing level "External Users" per Default.